In recent years, significant advances have been made in the development and implementation of mobile communications systems and devices with multimedia capability. For example, great strides have been made in the development and implementation of mobile phones and other types of mobile devices capable of transmitting and receiving audio media streams and video media streams in real-time. The 3G-324M protocol standard, which is a derivative of the International Telecommunications Union (ITU) H.324 standard, has been adopted to support such real-time streaming of audio and video media over circuit-switched wireless networks. Under the 3G-324M protocol standard, at least three different types of media streams can be defined, including audio, video, and control. To transmit such media streams over media channels within circuit-switched wireless networks, the 3G-324M protocol standard specifies the use of the H.223 multiplexing/de-multiplexing protocol, which can be employed to interleave or otherwise combine the different types of media streams, and, using the combined media stream, to assemble multiple multimedia packets into a single bit stream for subsequent transmission over a circuit-switched wireless network.
During the transmission of combined audio, video, and control media streams under the 3G-324M protocol standard, the audio media streams and the video media streams may, at times, need be separated into two or more media channels to allow certain processing operations to be performed that are dependent on the nature of the respective media streams. For example, such processing operations may include transforming the audio media streams and the video media streams to conform with/to a different protocol standard, changing the bandwidth used for the respective media streams, and/or performing other suitable processing operations on the respective media streams. While such processing operations are being performed on the audio media streams and the video media streams, the separate media channels carrying the media streams may pass through different processing elements, which may subject the respective media streams to different amounts of delay, resulting in a relative delay—referred to herein as “temporal skew”—between the two media channels. At a later point in time, some amount of delay may be applied to the so-called “faster” media channel to place the audio media streams and the video media streams in proper temporal alignment, thereby synchronizing subsequent playback of the two media channels. Such proper temporal alignment of the audio media streams and the video media streams is also referred to herein as “proper lip synchronization” of the respective media streams.
Under the 3G-324M protocol standard, the H.245 control protocol may be employed to send control messages to endpoints participating in the transmission of audio media streams and video media streams. For example, H.245 control messages may be sent to the endpoints to provide indications of any known amounts of temporal skew between media channels carrying the respective media streams. Such endpoints may include mobile devices such as mobile phones, and the audio media streams and the video media streams may be transmitted during a call initiated by one of the mobile phones. Further, the mobile phones may pass such temporal skew indications to a host application controlling the call activity between the respective devices. Based on such temporal skew indications, the host application can apply an amount of delay to the faster media channel to synchronize playback of the two media channels on the respective devices. This technique of synchronizing the playback of audio media channels and video media channels has drawbacks, however, in that it generally only considers the total amount of temporal skew occurring end-to-end along transmission paths, without taking into account any specific amounts of temporal skew that may have been introduced by intervening processing elements disposed along such transmission paths. Moreover, some types of endpoints may be incapable of handling such control messages, essentially leaving no other way of mitigating the effects of temporal skew.
It would therefore be desirable to have systems and methods of adjusting synchronization of audio media streams and video media streams in 3G mobile communications systems that avoid at least some of the drawbacks of the mobile communications systems and devices described above.